I had a friend when I was at uni who was a professional proofreader, if I asked him to check an essay he'd send it back in an extraordinarily short time saying it was fixed. If I then asked what he thought of the essay, his response was invariably "I didn't READ it, you didn't say you wanted me to READ It. I just fixed it..."

He would have done an exceptionally good job a the proof reading type task and had no idea what the content was, unless asked to also read the text he was editing...